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Department of Computer Science and Electronics

2.2.2. USART

The Universal Synchronous and Asynchronous serial Receiver and Transmitter

(USART) is a highly flexible serial communication device.

Figure 2-4 illustrate USART’s communication frame format.

The usual form of serial communication, and concretely the one that is

discussed, is the asynchronous serial communication. It is asynchronous in the senses

that a common clock signal is not required at both the transmitter and the receiver in

order to synchronize the data detection. [6]

A frame starts with the start bit followed by the least significant data bit. Then

the next data bits, up to a total of nine, are succeeding, ending with the most significant

bit. If enabled, the parity bit is inserted after the data bits, before the stop bits. When a

complete frame is transmitted, it can be directly followed by a new frame, or the

communication line can be set to an idle (high) state. Figure 2-4 illustrates the possible

combinations of the frame formats. Bits inside brackets are optional.

The frame format used by the USART is set by the UCSZ2:0, UPM1:0, and

USBS bits in UCSRB and UCSRC. The Receiver and Transmitter use the same setting.

Note that changing the setting of any of these bits will corrupt all ongoing

communication for both the Receiver and Transmitter.

The USART Character SiZe (UCSZ2:0) bits select the number of data bits in the

frame. Moreover he USART Parity mode (UPM1:0) bits enable and set the type of

parity bit. The selection between one or two stop bits is done by the USART Stop Bit

Select (USBS) bit. The receiver ignores the second stop bit.

Figure 2-4. USART frame

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